1. Field of the Invention
The present invention relates to a spool valve for a hydraulic valve actuator for an engine. More specifically, the present invention relates to a spool valve for a hydraulic valve actuator for an engine having a lightweight construction for a high speed of response.
2. Description of the Prior Art
It is well known in the prior art to utilize a spool valve to control the flow of high pressure hydraulic oil to a cavity containing a piston. This method of controlling the flow of hydraulic oil minimizes the response time required by minimizing friction and the distance required for opening and closing hydraulic porting. The material used for the hydraulic spool valve can be and has traditionally been a steel or aluminum metal with a homogeneous metallurgical construction. The problem with a lightweight aluminum spool valve is that the material tends to gall if excess friction or wear is encountered in any part of an operating bore and its nonmagnetic property does not lend itself to magnetic actuation. A steel material has enhanced anti-galling properties, however, the material density is high resulting in a high overall weight for the valve which must be axially moved at a high rate of acceleration by an actuator. The relatively high weight of the steel spool valve results in a slower response time and a larger actuator must be used with high power consumption and increased size.
Another problem with the prior art has been due to the phenomena known as "stiction" which occurs when a hydraulic film acts between two relatively close surfaces to resist motion between the surface in any direction. The surface tension of the oil film causes a relatively high shear force which resists motion of the spool valve. Prior art spool valves have encountered actuation delay due to the stiction phenomena. One solution has been to form pockets in a surface of the spool valve that reacts with the valve bore which are commonly called castellation channels. The castellation channels reduce the stiction by changing the characteristics of the hydraulic interface between the spool valve and the valve bore in the actuator body.
Another problem here-to-date with prior art spool valves has been in the balancing of hydraulic pressure that the spool valve is subjected to when controlling the flow of high pressure hydraulic oil. Unbalanced hydraulic forces can result in unwanted motion of the spool valve resulting in undesired actuator travel.